1. Field of the Invention
The present invention relates to an elevation null command generator suitable for use in equipment such as airborne monopulse radar, missile guidance systems, terrain avoidance radar systems, and the like, and to novel systems made possible by the use of the elevation null command generator. More particularly, the invention relates to an elevation null command generator (hereinafter ENCG) which provides an accurate means of directing the elevation monopulse null plane of a radar antenna at a patch of ground defined by a range signal generated within the radar or its associated equipment.
2. Description of the Prior Art
In terrain following-terrain avoidance radars the elevation monopulse null of the radar is stabilized by a device which consists of a single range gate generator and associated circuitry for averaging the output of the range gate to hold the mean signal in the elevation difference channel to zero or some other preset value. The accuracy of such systems is reduced by their susceptibility to bias effects of strong targets off the monopulse null but close enough to it to be within the range gate. Efforts to reduce this effect by narrowing the time width of the range gate result in decreasing the signal to noise ratio of the system, thereby again impairing accuracy. The device of the present invention is capable of giving results which are at least an order of magnitude improvement in the pointing accuracy of the ENCG as compared to such existing circuits.
3. Related Applications
The following copending patent applications, all of which are assigned to the same assignee as the present application, are illustrative of the type of system in which and of the associated equipment with which the elevation null command generator of this invention may be used. These applications and the references of record therein may also comprise additional prior art relevant to certain aspects of the present invention.
U.S. Pat. No. 736,932 filed on Jun. 5, 1968 in the name of William M. Bleakney entitled “Error Sensing Generator in a Monopulse Radar System” relates to an improved, airborne target tracking, monopulse radar system which includes an azimuth null command generator designed to provide an error signal which represents the angle between a command target direction and the antenna's actual monopulse azimuth null plane direction. The generator is supplied with radar return signals on sum and difference channels and with a frequency which represents a selected target in the sense that this frequency is equal to the doppler frequency shift for a target having a predetermined azimuth angular relationship to the direction of the vector velocity of the airborne system. The azimuth error signal in the disclosure is produced by first differentiating the sum channel signals and then subtracting the differentiated signals from the difference channel signal in a dual receiver. The difference output is then multiplied with properly phased sum channel signals to produce the desired error signal.
U.S. Pat. No. 666,542 filed on Aug. 29, 1967 in the name of F. C. Williams entitled “Monopulse Radar System” relates to an azimuth null command generator comprising a doppler frequency controlled beam pointing system for positioning the azimuth null plane of a monopulse antenna at a selected target. In this system a target azimuth position in the form of a doppler frequency signal is mixed with radar return signals in a sum and difference monopulse receiver. The output signals in the range gate controlled vicinity of a defined target are coupled to a processor where they are compared with the doppler frequency corresponding to the command azimuth of the selected target and where an error signal is derived which is indicative of the difference between the selected target azimuth position and the existing azimuth of the antenna monopulse null. The error signal is applied to an antenna drive servo which physically moves the antenna until the monopulse null is pointed at the selected target. The circuitry of the doppler beam pointer includes a center filter and a plurality of pairs of filters, the members of each pair of filters being equally spaced respectively above and below the center frequency at the monopulse null so that a cross product of received ground target signals may be derived by suitable multipliers to transform the energy contained in non-symmetrical backscatter to symmetrical form and thereby increase the accuracy of the null command generator. An azimuth null command generator of such a type wherein the output of a bank of filters is used to provide the factors for a cross product thus makes possible a tracking system usable in a monopulse doppler radar which operates independently of backscatter variations and which can therefore precisely track a weak ground target.
U.S. Pat. No. 163,535 filed Jul. 12, 1971 in the name of William M. Bleakney entitled “Airborne Missile Guidance System” relates to an airborne radar system with a monopulse antenna which system is also provided both with target area mapping means and with missile tracking and guidance means. The system receives return signals from a radar illuminated ground area including a target and in conjunction with a synthetic array data processor displays the illuminated area in terms of azimuth angle as measured by doppler frequency as one coordinate and in terms of range to the target as the other coordinate. Range and azimuth cursors are generated and displayed on the synthetic array map for target selection. These cursors represent controllable command values of range and doppler frequency. Manual means are provided to control the displayed cursors so that when the target is displayed under the cursors' intersection they indicate its doppler frequency and range. The command doppler frequency so derived is used to adjust the monopulse azimuth null plane of the antenna to point to the target. The missile guidance system disclosed in this application utilizes the fact that the locus of all points having the same azimuth angular deviation from the vector velocity of the airborne system, and hence the same doppler frequency shift, lies on a cone having its apex at the airborne system and having its axis of symmetry coinciding with the vector velocity of the system. Furthermore, the locus of all points having an equal range from the system at any given instant lies on a sphere centered at the system and having a radius equal to the specified range. The intersection of the iso-doppler cone and the iso-range sphere is a circle which will intersect the ground plane on which a target may be located at only two points. The coordinates of these points are given by azimuth angles having opposite signs and the same magnitude and by a range of the specified magnitude. The location of a target can thus be specified in terms of coordinates comprising a signed azimuth angle and a specified range. The cursors generate these values to indicate target location and the missile guidance system controls the missile to fly in the servo controlled azimuth monopulse null plane initially at a high horizontal elevation profile until the missile reaches the command range. At that time the missile is controlled to fly down the intersection of the azimuth null plane and the iso-range sphere so that it descends in a circular arc trajectory to a specified target. A system is thus provided which is capable of delivering a missile to a weakly reflecting target from an aircraft which flies at a safe standoff distance from the target. The side looking radar permits utilization of the missile trajectory described. This system may use the azimuth null command generators of either of the two previously mentioned applications, but it is intended for use purely against targets having a fixed position on the ground and does not contemplate servo controlled tracking of the antenna with respect to elevation. The possible trajectories are thus limited by the necessity of using the iso-range sphere for terminal missile guidance.
Systems of this type which use synthetic array radars for mapping to derive command signals have been referred to by the acronym SARCALM for Synthetic Array Radar Commanded Air Launched Missile.